exports.done = false
console.log('.....b1.....')
var a = require('./a.js')
console.log('.....b2.....')
console.log('在 b.js 之中，a.done = %j', a.done)
console.log('.....b3.....')
exports.done = true
console.log('.....b4.....')
console.log('b.js 执行完毕')